1. Field of the Invention
This invention is related to gas detection and monitoring, and more particularly to a system and method which utilize an electronically monitored gas sensor system to automatically detect, measure concentration, and announce the presence of toxic gaseous substances.
2. Background Art
Gas sensors haven been known and used in systems to detect the presence of toxic gases for a number of years in connection with a wide variety of applications, including residential and recreational vehicle use. Detectors are desirable in such uses due to the fuel combustion bi-products that enter the occupancy spaces thereof. A gasoline burning engine, a residential gas water heater, or furnace are examples of combustion that produce toxic gases.
A typical toxic by-product of fuel combustion is carbon monoxide gas (CO). When fuel that contains carbon is combusted, there will be a generation of carbon monoxide. The presence of CO in combustion is often accompanied by odors characteristic of the combustion process which may be pleasant, such as incense, resin in burning logs, or tobacco products. However, CO is odorless and tasteless. Additionally, CO is highly reactive, burns easily, is explosive, and is lethal to human beings. For good reasons, the detection of CO in high concentration levels is desirable to meet safety concerns.
In order to properly protect human beings from CO, a gas detection system must be capable of performing several functions. First, it must be able to detect carbon monoxide. Second, it must be able to deal with the information gathered by the gas detection system in a way that effectively balances the need to alarm or warn as to the presence of dangerous levels of CO, while also preventing false warnings and alarms. A gas detection system giving false alarms tends to lower the dependance and reliance of those being warned. To minimize these false alarm events, a detector must be provided that is effective to detect and warn as to truly hazardous conditions. Otherwise, those who are being falsely alarmed will eventually ignore its alarms or will disconnect the detector because of an unwillingness to tolerate the confusion of false alarms. For example, if a detect 6r gave a long and loud audible alarm whenever a toaster toasted bread or a fire was made in a fire place, the tolerance for such noise may be limited.
To be effective, a gas detection system must be capable of detecting CO in the concentration range and duration thereof which are of interest to human safety, while filtering out brief periods of relatively high gas concentration levels which may be of no consequence. Typically, an effective gas detection system must be able to detect at least 50 parts per million CO in the air. To avoid false alarms as to transient or momentary high levels of CO, the effective gas detection system must also determine a length or time of exposure to a hazardous CO level. Thus, a gas detector system that detects both the concentration of CO and the duration of exposure to hazardous concentrations of CO will accumulate a proper amount of information in order to warn those being exposed, rather than to set off an alarm each time any high level of CO concentration is detected.
High levels of CO are not necessarily dangerous when the exposure period is transient or momentary. For instance, it is not immediately harmful for a human being to breathe a large concentration of carbon monoxide such as 1,000 ppm for one or two breaths. Thus, a gas detector system that sets off an alarm when such a brief concentration is present in the air, even if it is present only for 10 seconds, is a gas detection system that is less effective psychologically to the user thereof.
High concentration exposures to CO are typical in every day events, such as taking a couple of breaths while walking behind a car that is not properly tuned. Improperly tuned automobiles produce a large amount of CO. Another example of possibly high CO generation is lighting a fire in a fireplace or lighting a charcoal grill. Determining a harmful exposure to CO involves both concentration level and the length of time there will be an exposure thereto.
In the past, others have made efforts to detect CO in the air with gas detection systems which put out an alarm for a concentration of both high concentration level and duration. Highly accurate systems prove to be expensive and impractical, such as laboratory gas analysis equipment. It would be desirable to provide an effective gas detection system that costs a fraction of the cost of laboratory gas detection equipment.
Semiconductor devices which are capable of detecting CO are becoming less expensive. Such semiconductors have a sensor element that is heated by a heater element to a predetermined temperature. The predetermined temperature is temperature at and above which the gas that is to be detected will catalyze. Different gases catalyze at different temperatures when in fluid contact with the sensor element that has been heated by the heater element to a gas-specific temperature. When the sensor element is heated to the gas-specific temperature, the gas will catalyze in a reaction with the exposed surface areas of the sensor element causing it to conduct electricity somewhat proportionally, but non-linearly, to the concentration level of the gas. Electrically, the resistivity or the resistance of the sensor element gets lower as the concentration gets higher. The current through the sensor element gets higher as the concentration level gets higher. Stated otherwise, resistivity of the sensor element increases as the concentration level decreases in an inversely proportional but non-linear relationship.
While CO detecting semiconductors have a long operational life, they also may yield false alarms due to the presence of extraneous gases, due to humidity, or due to uncompensated sensitivity of a sensor element to ambient temperature which causes the signal produced therefrom to be skewed.
In order to measure concentration levels of a particular gas, such as CO, the sensor element should be heated to the gas-specific temperature. As the sensor element cools down in a cyclic excitation, impurities also adhere to the surface of the sensor element. When kept at a constant high temperature, the impurities do not tend to build up because they will catalyze by adsorption. Thus, it is advantageous to heat the sensor element to an elevated temperature, above the predetermined gas-specific temperature for measuring gas concentration levels, in order to catalyze the impurities off of the sensor element.
Electrical power is modulated as it is applied to the sensor heater element so as to heat up and cool down the sensor element to cyclically measure gas concentration levels and catalyze off impurities. To measure the concentration level of the gas desired to be detected, an electrical potential is applied to the sensor element while it is heated to the gas-specific temperature and then the change in current is measured which corresponds to the concentration level of the gas. The greater the thermal mass of the sensor element being cyclically heated and cooled, the longer the heat-up and cool-down cycle, the longer the time between gas concentration readings, and the more power consumption needed to raise the temperature of the sensor element while accurately measuring gas concentration by catalyzing impurities. By increasing the number of such reading and catalyzing cycles over a period of time, a more accurate assessment of the dynamic nature of toxic gas concentration levels can be made. Thus, it would be an improvement in the art to maximize power efficiency by minimizing the thermal mass of the sensor element, minimizing the heat-up and cool-down cycle time, and maximizing the number of readings of gas concentration levels over a period of time.
Gas detectors systems whose purpose is to protect against human hazards have their goal to keep the level of CO that is inhaled by human to a safe amount. Inhaled CO becomes COHb in the blood of an inhaling human. COHb in the blood is a function of both CO concentration and inhaling over time. Mathematical algorithms to calculate COHb can be analytical, complex analytical, or numerical. Each of such algorithms, when used in real time gas detection systems, involve varying amounts of data, processing speed and power. It is desirable to keep processor costs down and speeds up by using algorithms that give good numerical approximates of COHb with minimal processing power so as to avoid data processing hardware costs, while still accomplishing gas detection having the aforementioned safety standards and minimized false alarms.
Having a properly calibrated gas detection system is important to human safety. Calibration equipment can be expensive and may mitigate against efforts to keep the cost of a gas detection system down. Accordingly, it is desirable to minimize the cost of calibrating a gas detection system.
Another cost related aspect of gas detection systems is the way in which visual indications of concentration levels and visible alarms of toxic concentrations of gas are presented to an observer of the gas detection systems. Visible meters and gauges, either having analog or digital display means, tend to be costly and could mitigate against wide spread purchase and use of such detection systems. Accordingly, it would be an advance in the art to provide a lost cost visible gas detection alarm. One such an application in need of CO gas detection is in recreational vehicles where fuel combustion is taking place.